$ D = \left[\begin{array}{rrr}1 & 2 & 1 \\ 2 & 0 & 0 \\ 0 & 0 & 2\end{array}\right]$ What is $ D^{-1}$ ?
Explanation: $ = \left[\begin{array}{rrr}0 & \frac{1}{2} & 0 \\ \frac{1}{2} & -\frac{1}{4} & -\frac{1}{4} \\ 0 & 0 & \frac{1}{2}\end{array}\right]$